【作者】 姜山；

【导师】 刘天模；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2011， 博士

【摘要】 本文对具有挤压织构和压缩织构的AZ31镁合金进行了组织性能测试,实验结果表明孪生对AZ31镁合金的各向异性及拉压不对称性具有重要的影响。为了深入了解镁合金孪生变形机理,本文建立了一套全新的原子模型来描述镁合金的{1012}孪生、{1011}孪生和{1013}孪生过程中原子的运动规律,并在此基础上提出了3个用来衡量镁合金孪生发生难易的新参数:第一个是QPAG单元的旋转角度α;第二个是QPAG单元之间的相对滑移量λ;第三个是孪生过程中原子需要克服的原子变形量ε。这三个参数中的λ和ε可以作为衡量孪生发生难易的参数,该参数越大,孪生越难发生,这与实验结果是一致的。这两个参数作为衡量孪生发生难易的参数不是偶然的,而是具有实际的物理意义的,因为较大的QPAG单元滑动量和原子变形量意味着孪生的激活需要克服更高的应力。此外,QPAG原子群模型还可以很好地解释关于孪生的实验现象,如孪晶长大、退孪生以及孪生再结晶等。根据这些研究结果主要得到如下结论。①对在723K下进行5h的均匀化的AZ31镁合金试样进行不同变形量的室温压缩实验,发现室温压缩产生的孪生组织随着变形量的增加而增加,形态也发生变化,孪晶的形状由最初的直而窄发展到宽而弯。对应变量0.1的室温压缩试样的EBSD分析表明,变形所获得孪晶绝大部分为{1012}拉伸孪晶,只有极少量的{1011}压缩孪晶。对退火后的室温压缩样进行组织观察发现在孪晶内部部析出了细小的再结晶晶粒。②首次通过建立QPAG单元模型,揭示了AZ31镁合金孪生过程中原子的运动规律。研究发现孪生中原子的运动可以归结为QPAG单元的旋转,由于在QPAG单元内部不存在孪生阻力,所以可以认为孪生的阻力主要来自于QPAG单元之间的相对运动。在{1012}孪生中QPAG单元旋转15.9°,相邻的QPAG单元在界面处会发生0.349a的相对位移。③在{1012}孪生中建立的QPAG模型也适用于描述{1011}孪生中的原子运动,在{1011}孪生中原子的运动仍然可以归结为QPAG单元的旋转,但是在{1011}孪生中存在两种运动模式不同的QPAG单元:一种只是简单地在y-o-z面内旋转;另一种不仅在y-o-z面内旋转,还要沿y轴移动0.5a的距离。这两种单元呈现出交替分布的特点,他们在孪生中都旋转14.7°,单元之间相邻原子的相对位移量为0.517a。尽管{1011}孪生中的QPAG单元旋转角度小于{1012}的,但由于其第二类QPAG单元的出现,致使其原子的运动复杂程度大大提高,进而导致了单元原子相对滑移量的增加,这是导致最终{1011}孪生较{1012}孪生难发生的主要原因。④QPAG模型还适用于描述{1013}孪生中的原子运动。在{1013}孪生中与{1011}孪生相似,也存在两种交替分布的QPAG单元:一种只是简单地在y-o-z面内旋转;另一种不仅在y-o-z面内旋转,还有沿旋转轴向平移0.5a的距离。但不同的是{1013}中的QPAG单元与{1011}中的呈“手性”关系。这两种QPAG单元在孪生过程中都只旋转6.3°,但邻近单元之间原子的相对位移量为0.539a,高于{1011}孪生的0.517a,这可能是其较{1011}孪生更为少见的主要原因。⑤镁合金孪生中QPAG原子群单元的旋转具有传递性。镁合金在孪生的过程中,其原子需要克服一定的弹性形变,该原子弹性形变存在一个孪生临界值,在形变达到临界值之前,撤消应力原子回到初始位置,材料不发生塑性变形;当变形超过临界值时,孪生瞬间发生。镁合金基体中的层错原子在孪生后的孪晶区因无法到达正常的格点位置而以混排原子的形式出现,这些混排原子处于热力学上的高能量状态,容易成为再结晶的形核点。⑥镁合金中的孪晶界可以看做是由位向处于基体和孪晶之间的渐变QPAG原子群单元组成。孪晶界可以通过QPAG单元的旋转实现垂直于其法向的移动:当QPAG单元的旋转使晶界向基体方向运动时,孪晶长大;当QPAG单元的旋转使晶界向孪晶方向运动时,发生退孪晶。镁合金的基面滑移总是伴随着晶粒转动和晶界运动。由于孪晶界的特殊的原子排列使孪晶界滑移和伸缩难以实现,当基体和孪晶处于基面滑移的应力软取向时,孪晶界附近将发生位错积聚,成为断裂源的增殖区。⑦在{1012}孪生,{1011}孪生和{1013}孪生中,原子需要克服变形量分别为0.024,0.031和0.038。这与AZ31镁合金室温压缩时所观察到的产生的孪晶绝大多数为{1012}孪晶,只有极少数为{1011}孪晶,而未发现{1013}孪晶的实验结果是一致的。这说明影响镁合金孪生变形难易程度的本质因素是孪生过程中原子需要克服的变形量。更多还原

【Abstract】 Twinning deformation is considered an important factor affecting the anisotropy and the asymmetry of the AZ31 magnesium alloy through the tests of microstructures and properties of the material with extrusion textures and compression textures. To understand the mechanism of twinning deformation of magnesium alloys, a new model was established to describe the atomic motion in {1012} twinning, {1011} twinning and {1013} twinning, and on the basis of which three new parameters have been proposed to measure the difficulty degree of twinning: The first parameter is the rotational angle of the QPAG unitsα; the second parameter is the relative displacement magnitude between the adjacent QPAG unitsλ; the third parameter is the atomic deformation to overcome during twinningε. The later two parametersλandεcan be used to measure the difficulty degree of twinning; the lager these parameters are, the harder the twinning occurs, which is consistent with the experiment results. It is not accidental that these two parameters can be used to measure the difficulty degree of twinning, because they are of true physics meaning for that the larger value of the relative displacement magnitude and the atomic deformation means the higher stress to overcome. In addition, the QPAG model also can be used to explain the interesting phenomenon about twinning, such as twin growth, detwinning and twin recrystallization. According to these results some conclusions have been drawn about the deformation behavior and the twinning mechanism of AZ31 magnesium alloy.①The AZ31 magnesium alloy samples homogenized at 723 K for 5 h were compressed to various strains at room temperature. The results show that the amount of twins is increased with the deformation, and the twin shape turn wider and bended from the original shape thin and straight. EBSD technique show that most of the twins are {1012} extension twins, and only a few of them are {1011} contraction twins. Static recrystallization occurred in the twins when the compressed samples were annealed with small recrystallized grains generated within the twins.②The QPAG unit model was established for the first time to reveal the law of atomic motion during twinning in AZ31 magnesium alloy. The study indicates that the atomic motion during twinning can be ascribed to the rotational motion of the QPAG units. Since there is no resistance within interior of the QPAG units, it can be considered that the twin resistance mainly comes from the relative motion between the adjacent QPAG units. The QPAG units rotate an angle of 15.9°during {1012} twinning, and the relative displacement magnitude between the adjacent QPAG units is 0.349a.③The QPAG unit model established in {1012} twinning is also suitable for describing the atomic motion in {1011} twinning, the atomic motion in {1011} twinning also can be ascribed to the rotational motion of the QPAG units. However, there exist two kind of QPAG units: one of them (type I) rotated only in the plane y-o-z;the other one (type II) not only rotate in the plane y-o-z, but also shift along the x axis for 0.5 a. These two kind of QPAG units are distributed alternatively. Both of these two kind of QPAG units rotate an angle of 14.9°during {1011} twinning, and the relative displacement magnitude between the adjacent units is 0.517a. Though the rotational angle in {1011} twinning is smaller than in {1012} twinning, the appearance of the type II QPAG units make the atomic motion in {1011} twinning become more complex and the relative displacement magnitude larger, which may be the essential reason that {1011} twinning is harder to occur than {1012} twinning.④The QPAG unit model is also suitable for describing the atomic motion in {1013} twinning. There exist totally two kind of alternately distributed QPAG units in {1013} like in {1011} twinning: one of them (type I) rotated only in the plane y-o-z;the other one (type II) not only rotate in the plane y-o-z, but also shift along the x axis for 0.5 a, the difference is that the QPAG units in {1013} twinning represent a“chiral”relation with that in {1011} twinning. Both of these two kind of QPAG units rotate an angle of only 6.3°during {1013} twinning. However, the relative displacement magnitude between the adjacent units is 0.539a, larger than the value in {1011} twinning, this may be the reason that {1013} twins are more infrequent to be observed than {1011} twins.⑤The rotational motion of the QPAG units during twinning has a property of transfer. The twinning atoms are required to overcome a certain deformation during twinning, and there exist a critical value about the deformation to overcome. When the deformation is before the critical value, the twinning atom will return to the original position once the stress is removed;when the deformation pass the critical value, the twinning will occur. The stacking fault atoms can not reach the normal position in the twin after twinning, and they arranged in a disordered state. Thermodynamically, these disordered atoms are in a high-energy state, and thus inclined to become the recrystallization sites. ⑥The twin boundaries can be regarded as composed of the QPAG units oriented between the matrix and twin gradually. The twin boundaries can move along the norm direction through the rotation of these QPAG units: twin growth occurs when the rotation of the QPAG units make the twin boundaries move to the matrix;detwinning takes place when the rotation of the QPAG units make the twin boundaries move to the twin. Basal slip in the magnesium always results in the grain rotation and the grain boundary movement. Because the twin boundary sliding is difficult to occur for the special atomic arrangement of the twin boundaries, dislocations will accumulate at twin boundary when the twin and matrix in a soft orientation, and thus the twin boundaries will become the nucleation zone of fracture.⑦The values of atomic deformation need to overcome in {1012} twinning, {1011} twinning and {1013} twinning are 0.024, 0.031 and 0.038, respectively. The order of these value is consistent with the observation in the experiment of the room temperature compression of the AZ31 magnesium alloy, that is, the {1012} twins are the easiest to form, followed by {1011} twins, and the {1013} twins are the hardest to form. In conclusion, the factor that determine the difficulty degree of twinning to occur is the atomic deformation need to overcome during twinning.更多还原